1. Field of the Invention
The present invention relates to a method of and apparatus for eliminating interblock distortion appearing in a reproduced image which is produced on the basis of compressed image data obtained for each of a plurality of pixel blocks dividing an original image.
2. Description of the Prior Art
An image processor such as a process scanner generally reads an original image to obtain image data for every pixel which is a unit region extremely small compared with the original image. Therefore, if the image data of the original image is stored for every pixel, a great amount of storage capacity is required. In order to cope with such a problem, an original image is divided into a plurality of blocks, whereby image data is compressed for every block in order to reduce storage capacity required for the image data, as is well known in the art.
FIG. 1 is an explanatory diagram showing pixel blocks of an original image. Referring to FIG. 1, an original image 20 is divided into Y, where Y is an integer, along the main scanning direction i and into X, where X is an integer, along the subscanning direction j, to consist of (X.times.Y) pixels P. It is to be noted that FIG. 1 partially illustrates lines dividing the original image 20 into the pixels. The original image 20 is also divided into a plurality of pixel blocks B.sub.ij each of which has a plurality of pixels. Namely, the original image 20 is divided into im along the main scanning direction i and into jm along the subscanning direction j, to have (im.times.jm) pixel blocks B.sub.ij. FIG. 2 illustrates arrangement of pixels P.sub.mn in one pixel block B.sub.ij. Referring to FIG. 2, the pixel block B.sub.ij is divided into M along the main scanning direction i and into N along the subscanning direction j, to have (M.times.N) pixels P.sub.mn. The subscripts m and n indicate coordinate positions of the pixels in one pixel block B.sub.ij, where the subscript m is associated with the main scanning direction i and the subscript n is associated with the subscanning direction j.
Data compression of image data is performed, for example, by firstly performing orthogonal transformation such as cosine transformation or Hadamard transformation on density distribution of pixels P.sub.mn for every pixel block B.sub.ij and secondly preserving coefficients obtained by the transformation. The method of the data compression is generally called image encoding, and the number of compressed image data with respect to one pixel block B.sub.ij is not more than the number (M.times.N) of the pixels P.sub.mn included in the pixel block B.sub.ij. Particularly, when density is gently changed in the pixel block B.sub.ij, compressed image data often consists only of a value of average density in the pixel block B.sub.ij. FIG. 3 is an explanatory diagram showing three compressed image data which consist only of respective average densities. Referring to FIG. 3, the horizontal axis is the main scanning direction i shown in FIG. 1, and the vertical axis is density value D. FIG. 3 shows density distribution g.sub.0 through three pixel blocks B.sub.i-1j, B.sub.ij and B.sub.i+1j which are adjacent to each other along the main scanning direction i and compressed image data A.sub.i-1 to A.sub.i+1 obtained for respective pixel blocks. Since the density distribution g.sub.0 of the original image is gently changed, the compressed image data A.sub.i-1 to A.sub.i+1 consist only of respective average densities.
In this specification, the term "density" denotes not only optical density, but also other values corresponding to optical density such as Munsell value, output signal level of an image reader, halftone area rate in halftone image recording and the like.
Thus, the compressed image data A.sub.i-1 to A.sub.i+1 consisting only of respective average densities properly express the density distribution g.sub.0 of the original image through the respective pixel blocks B.sub.i-1j to B.sub.i+1j. However, interblock distortion, which is a stepwise difference of density, may appear on a boundary between each pair of neighboring pixel blocks. Referring to FIG. 3, differences d and d' appear in boundaries on both sides of the pixel block B.sub.ij. When the image is reproduced in this state, the difference d and d' are recognized as deterioration in picture quality. Although the interblock distortion is caused as the result of compression of image data, data compressibility decreases if data compression technique or data encoding technique which substantially causes no interblock distortion is applied. Further, these techniques cannot achieve objects such as reduction of storage capacity and facilitation of data transmission because of the low compressibility.